Hydrocharitaceae

Vallisneria spiralis, copyright Lamiot.

Belongs within: Alismatales.

The Hydrocharitaceae are a group of perennial herbs living at least partially submerged in water that are more diverse in warmer regions of the world. Members of the genera Enhalus, Thalassia and Halophila are marine.

Flowers in the water
Published 9 March 2009
Hydrocharis morsus-ranae, frogbit (and is there anyone out there who can tell me why this plant is called “frogbit”?), an insect-pollinated example of Hydrocharitaceae. Photo by J. R. Crellin.

The Hydrocharitaceae are a small but morphologically diverse family of fully aquatic monocots. About eighty species are included in about fifteen genera, excluding the genus Najas which has occasionally been suggested to belong in this family (Tanaka et al. 1997). Hydrocharitaceae includes both freshwater and marine species, such as seagrasses (Enhalus, Thalassia and Halophila), eelgrasses (Hydrilla, Vallisneria) and oxygen weeds (Egeria, Elodea, Lagarosiphon). Depending on the species, Hydrocharitaceae may live their lives partially or entirely submerged.

A major factor in the diversity of this family derives from the diversity of pollination methods. Hydrocharitaceae may be monoecious (separate male and female flowers, but on the one plant) or dioecious (separate male or female plants). Many of the genera in this family are insect-pollinated (entomophilous), with flowers protruding from the water, and comparison with related families suggests that this is the ancestral condition for the family. However, more that one lineage of Hydrocharitaceae has evolved to take advantage of their home environment by becoming water-pollinated (hydrophilous). Such species may be epihydrophilous, with flowers borne at the water surface, or hypohydrophilous, with flowers completely underwater. The genera Vallisneria, Lagarosiphon, Nemachandra and Enhalus have become epihydrophilous in a way that no other aquatic flowering plant has—rather than releasing pollen into the water like other hydrophilous plants, the entire male flower is released to float on the surface of the water until it reaches a female flower. Remarkably, while this process is unique to Hydrocharitaceae, genera with detaching flowers do not form a single clade. Instead, the process has evolved at least three times within the Hydrocharitaceae (Tanaka et al. 1997, who did not test the position of Nemachandra).

Inflorescence of the seagrass Enhalus acoroides, and a floating congregation of the small detached male flowers. Photo from Team Seagrass.

Hydrocharitaceae also have other reproductive options open to them. Vegetative reproduction is common among plants, a factor which gardeners have profited from for generations. The Hydrocharitaceae are vegetative reproducers par excellence, with broken-off fragments all too ready to reroot and establish themselves. In the case of the Florida seagrass Halophila johnsonii, the only marine plant listed as endangered in the United States, vegetative reproduction may be the only thing keeping it going. Only female flowers have ever been recorded for this species, and seed production has never been recorded. While some have suggested that Halophila johnsonii may reproduce apomictically (parthenogenetically for the zoologically-inclined), York et al. (2008) demonstrated that this is probably not the case—ovules and gametes are produced in the same manner as other sexually-reproducing Halophila species. Theoretically, pollination of these ovules should be entirely possible—but somehow, the males have all disappeared, and the Halophila johnsonii females are waiting for a pollinator that will never come.

Elodea canadensis, one of the fully submerged “oxygen weeds” widely used in fish tanks, from whence they escape to take over the world. Photo by Ondřej Zicha.

Unfortunately, this propensity for vegetative propagation is also the dark side of Hydrocharitaceae. Widely propagated as aquatic ornamentals or for aquaria and fish tanks, Hydrocharitaceae have found it all too easy to escape their alloted positions and invade exotic waterways. In New Zealand, three species of oxygen weed (Egeria densa, Lagarosiphon major and Elodea canadensis) have become leading invasives, despite the fact that, for all three, individuals of almost invariably only a single sex are present in the wild, rendering sexual reproduction nonexistent (Healy & Edgar 1980). In 1968, power production at a hydroelectric station on the Waikato River was brought to a halt by dense Lagarosiphon major blocking the station’s water intake. Elodea canadensis achieved even greater invasions:

The heavy stand in Lake Rotoroa, Nelson Lakes National Park is noteworthy: between 1965 and 1971 the plant formed a virtually complete marginal weed-bed to a depth of 8.5 m, with stems to 6 m high. Here, in terms of dry matter per m2, the amount of weed herbage is significantly higher than that recorded for any other freshwater macrophyte community elsewhere in the world. (Healy & Edgar 1980)

Systematics of Hydrocharitaceae

Characters (from Healy & Edgar 1980): Aquatic perennials of fresh or salt water, partly or completely submersed; monoecious or dioecious. Stems long, slender, simple or branched, or 0. Leaves basal or cauline, usually sessile, alternate or whorled, variable in shape and size, submersed or floating. Flowers actinomorphic, commonly unisexual, sometimes bisexual. Inflorescence (or flower) sessile or long-pedunculate, in a tubular bifid spathe or within 2 opposite bracts; male flowers usually many within spathe; female flowers solitary; bisexual flowers solitary or several together. Perianth-segments usually 6 or fewer, in 1–2 whorls, 3 (rarely 2) in each whorl; tube often elongate. Stamens (1)–3–many, anthers 2-celled, opening by longitudinal slits (staminodia in female flowers). Ovary inferior, syncarpous, 1-locular, with 3–6 (rarely more) parietal placentae (rudimentary ovaries in male flowers); style-branches as many as placentae; stigmas entire or bifid; ovules many. Fruit submersed, often fleshy, indehiscent, occasionally rupturing irregularly. Seeds many.

<==Hydrocharitaceae [Hydrocharideae, Hydrocharitales, Najadaceae]
|--Hydrocharis morsus-ranaeBO08
|--Lagarosiphon major (Ridley) Moss ex Wager 1928M30, HE80
|--Stratiotes aloidesM30, C55
|--XystrolobusM30
|--HydromystriaYY22
|--EnhalusP88
| |--E. acoroides (Linnaeus) Royle 1839HS14
| `--E. koenigiiT-W89
|--Egeria densa Planchon 1849HE80 [=Elodea densaH93]
|--Hydrocharites obcordatus Weber 1855CBH93
|--Cymodoceites parisiensis (Brongniart) Bureau 1886CBH93
|--Limnobium spongiaAn02
|--ElodeaH93
| |--E. canadensis Michaux 1803HE80
| `--E. nuttalliiH93
|--BoottiaM30
| |--B. aschersonianaM30
| `--B. muricataM30
|--HydrillaM30
| |--H. kaynensisJD05
| `--H. verticillataHE80
|--Thalassia Banks ex König 1805HL09
| |--T. hemprichii (Ehrenberg) Aschersson 1871 [=Schizotheca hemprichii Ehrenberg 1832]HL09
| `--T. testudinumK-M02
|--Halophila Thouars 1806HL09
| |--H. decipiens Ostenfeld 1902HL09
| |--H. minor (Zollinger) den Hartog 1957HS14
| |--H. ovalis (Brown) Hooker 1859JL09 [=Caulinia ovalis Brown 1810HL09; incl. H. ovataB78]
| |--H. spinulosa (Brown) Ascherson 1875HS14 [=Caulinia spinulosaB78]
| `--H. stipulaceaP71
|--BlyxaM30
| |--B. aubertiiLK14
| | |--B. a. var. aubertiiLK14
| | `--B. a. var. echinospermaLK14
| |--B. octandraLK14
| `--B. roxburghiiT-W89
|--OtteliaM30
| |--O. alismoidesP88
| |--O. minutissimaJD05
| |--O. ovalifolia (Brown) Richard 1811HE80
| | |--O. o. ssp. ovalifoliaLK14
| | `--O. o. ssp. chrysobasisLK14
| `--O. praeteritaS87
|--VallisneriaM30
| |--V. americanaB97
| |--V. annuaLK14
| |--V. giganteaHE80
| |--V. nanaLK14
| |--V. spiralisHE80
| | |--V. s. var. spiralisPP07
| | `--V. s. var. denseserrulataPP07
| |--V. stylosaJD05
| `--V. tripteraLK14
`--NajasH93
|--N. flexilisB97
| |--N. f. var. flexilisV72
| |--N. f. var. congestaV72
| `--N. f. var. robustaV72
|--N. foveolataLK14
|--N. gracillimaH93
|--N. gramineaLK14
|--N. guadelupensisH93
|--N. indicaC55
|--N. majorC55
|--N. malesianaLK14
|--N. marinaAb02
|--N. minorV72
|--N. olivaceaV72
|--N. pseudogramineaLK14
`--N. tenuifoliaLK14

*Type species of generic name indicated

References

[Ab02] Able, K. W. 2002. Killifishes. Family Fundulidae. In: Collette, B. B., & G. Klein-MacPhee (eds) Bigelow and Schroeder’s Fishes of the Gulf of Maine 3rd ed. pp. 292–297. Smithsonian Institute Press: Washington.

[An02] Anderson, R. S. 2002. Curculionidae Latreille 1802. In: Arnett, R. H., Jr, M. C. Thomas, P. E. Skelley & J. H. Frank (eds) American Beetles vol. 2. Polyphaga: Scarabaeoidea through Curculionoidea pp. 722–815. CRC Press.

[B78] Bentham, G. 1878. Flora Australiensis: A description of the plants of the Australian Territory vol. 7. Roxburghiaceae to Filices. L. Reeve & Co.: London.

[BO08] Bönsel, D., I. Ottich, A. Malten & G. Zizka. 2008. An updated list of the vascular plants of Frankfurt am Main (Pteridophyta & Spermatophyta). Senckenbergiana Biologica 88 (1): 111–121.

[B97] Brown, K. M. 1997. Temporal and spatial patterns of abundance in the gastropod assemblage of a macrophyte bed. American Malacological Bulletin 14 (1): 27–33.

[C55] Candolle, A. de. 1855. Géographie Botanique Raisonée: Ou exposition des faits principaux et des lois concernant la distribution géographique des plantes de l’époque actuelle vol. 2. Librairie de Victor Masson: Paris.

[CBH93] Collinson, M. E., M. C. Boulter & P. L. Holmes. 1993. Magnoliophyta (‘Angiospermae’). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 809–841. Chapman & Hall: London.

[HE80] Healy, A. J., & E. Edgar. 1980. Flora of New Zealand vol. 3. Adventive cyperaceous, petalous and spathaceous monocotyledons. P. D. Hasselberg, Government Printer: Wellington (New Zealand).

[H93] Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley (California).

[HL09] Huisman, J. M., F. Leliaert, H. Veerbruggen & R. A. Townsend. 2009. Marine benthic plants of Western Australia’s shelf-edge atolls. Records of the Western Australian Museum Supplement 77: 50–87.

[HS14] Huisman, J. M., & A. Sampey. 2014. Kimberley marine biota. Historical data: marine plants. Records of the Western Australian Museum Supplement 84: 45–67.

[JD05] Junge, F. W., M. Dolezych, H. Walther, T. Böttger, A. Kühl, L. Kunzmann, P. Morgenstern, T. Steinberg & R. Stange. 2005. Ein Fenster in Landschaft und Vegetation vor 37 Millionen Jahren: Lithologische, sedimentgeochemische und paläobotanische Befunde aus einem Paläoflusssystem des Weißelsterbeckens. Mauritiana 19 (2): 185–273.

[K-M02] Klein-MacPhee, G. 2002. Filefishes. Family Monacanthidae. In: Collette, B. B., & G. Klein-MacPhee (eds) Bigelow and Schroeder’s Fishes of the Gulf of Maine 3rd ed. pp. 592–598. Smithsonian Institute Press: Washington.

[LK14] Lyons, M. N., G. J. Keighery, L. A. Gibson & T. Handasyde. 2014. Flora and vegetation communities of selected islands off the Kimberley coast of Western Australia. Records of the Western Australian Museum Supplement 81: 205–244.

[M30] Moss, C. E. 1930. A new genus of Hydrocharitaceae from tropical Africa. Proceedings of the Linnean Society of London 141: 14–16.

[PP07] Pandey, R. P., & P. M. Padhye. 2007. Studies on phytodiversity of Arid Machia Safari Park-Kailana in Jodhpur (Rajasthan). Bulletin of the Botanical Survey of India 49: 15–78.

[P88] Polunin, I. 1988. Plants and Flowers of Malaysia. Times Editions: Singapore.

[P71] Por, F. D. 1971. One hundred years of Suez Canal—a century of Lessepsian migration: retrospect and viewpoints. Systematic Zoology 20 (2): 138–159.

[S87] Stephens, W. J. 1887. On the Biloela labyrinthodont (second notice). Proceedings of the Linnean Society of New South Wales, series 2, 1 (4): 1113–1121, pl. 14.

Tanaka, N., H. Setoguchi & J. Murata. 1997. Phylogeny of the family Hydrocharitaceae inferred from rbcL and matK gene sequence data. Journal of Plant Research 110 (3): 329–337.

[T-W89] Tenison-Woods, J. E. 1889. On the vegetation of Malaysia. Proceedings of the Linnean Society of New South Wales, series 2, 4 (1): 9–106, pls 1–9.

[V72] Voss, E. G. 1972. Michigan Flora. Part I. Gymnosperms and Monocots. Cranbrook Institute of Science and University of Michigan Herbarium.

[YY22] Yampolsky, C., & H. Yampolsky. 1922. Distribution of sex forms in the phanerogamic flora. Bibliotheca Genetica 3: 1–62.

York, R. A., M J. Durako, W. J. Kenworthy & D. W. Freshwater. 2008. Megagametogenesis in Halophila johnsonii, a threatened seagrass with no known seeds, and the seed-producing Halophila decipiens (Hydrocharitaceae). Aquatic Botany 88: 277–282.

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